Fiber optic networks are becoming increasingly popular for data transmission due to their high speed and high data capacity capabilities. Multiple wavelengths may be transmitted along the same optic fiber. This totality of multiple combined wavelengths comprises a single transmitted composite signal. A crucial feature of a fiber optic network is the separation of the optical signal into its component channels, typically by a wavelength division multiplexer. This separation must occur in order for the exchange of channels between signals on "loops" within networks to occur. The exchange occurs at connector points, or points where two or more loops intersect for the purpose of exchanging channels.
Add/drop systems exist at the connector points for the management of the channel exchanges. The exchanging of data involves the exchanging of matching channels from two different loops within an optical network. In other words, each signal drops a channel to the other loop while simultaneously adding the matching channel from the other loop.
FIG. 1 illustrates a simplified optical network 100. A fiber optic network 100 could comprise a main loop 150 which connects primary locations, such as San Francisco and New York. In-between the primary locations is a local loop 110 which connects with loop 150 at connector point 140. Thus, if local loop 110 is Sacramento, channels at San Francisco are multiplexed into an optical signal which will travel from San Francisco, add and drop channels with Sacramento's signal at connector point 140, and the new signal will travel forward to New York. Within loop 110, optical signals would be transmitted to various locations within its loop, servicing the Sacramento area. Local receivers (not shown) would reside at various points within the local loop 110 to convert the optical signals into the electrical signals in the appropriate protocol format.
The separation of an optical signal in the composite signal into its component channels is typically performed by a dense wavelength division multiplexer. FIG. 2 illustrates add/drop systems 200 and 210 with dense wavelength division multiplexers 220 and 230. An optical signal from Loop 110 (.lambda..sub.1 -.lambda..sub.n) enters its add/drop system 200 at node A (240). The signal is separated into its component channels by the dense wavelength division multiplexer 220. Each channel is then outputted to its own path 250-1 through 250-n. For example, .lambda..sub.1 would travel along path 250-1, .lambda..sub.2 would travel along path 250-2, etc. In the same manner, the signal from Loop 150 (.lambda..sub.1 '-.lambda..sub.n ') enters its add/drop system 210 via node C (270). The signal is separated into its component channels by the wavelength division multiplexer 230. Each channel is then outputted via its own path 280-1 through 280-n. For example, .lambda..sub.1 ' would travel along path 280-1, .lambda..sub.2 ' would travel along path 280-2, etc.
In the performance of an add/drop function, for example, .lambda..sub.1 is transferred from path 250-1 to path 280-1. It is combined with the others of Loop 150's channels into a single new optical signal by the dense wavelength division multiplexer 230. The new signal is then returned to Loop 150 via node D (290). At the same time, .lambda..sub.1 ' is transferred from path 280-1 to path 250-1. It is combined with the others of Loop 110's channels into a single optical signal by the dense wavelength division multiplexer 220. This new signal is then returned to Loop 110 via node B (260). In this manner, from Loop 110's frame of reference, channel .lambda..sub.1 of its own signal is dropped to Loop 150 while channel .lambda..sub.1 ' of the signal from Loop 150 is added to form part of its new signal. The opposite is true from Loop 150's frame of reference. This is the add/drop function.
Conventional methods used by wavelength division multiplexers in separating an optical signal into its component channels include the use of filters and fiber gratings as separators. A "separator," as the term is used in this specification, is an integrated collection of optical components functioning as a unit which separates one or more channels from an optical signal. Filters allow a target channel to pass through while redirecting all other channels. Fiber gratings target a channel to be reflected while all other channels pass through. Both filters and fiber gratings are well known in the art and will not be discussed in further detail here.
A problem with the conventional separators is their limitation to channels with identical spacings and/or bandwidths. However, networks typically transmit multiple signals with different modulations or data transfer rates. These different modulation rates lead to different effective bandwidths, which are herein referred to an information bandwidths. For instance, greater modulation rates are associated with occupation or utilization of greater optical bandwidth (defined either in frequency or in wavelength) than are slower modulation rates. However, optical components within conventional wavelength division multiplexed (WDM) optical communications systems are associated with certain fixed channel band pass widths. In WDM systems in which different signals are transmitted at different data transfer rates along different channels, the available fiber bandwidth can be utilized most efficiently when hardware bandwidths match the information bandwidths dictated by the channel data transfer rates. This requires optical hardware with uneven or asymmetric channel pass bands. Conventional channel separators have fixed channel bandwidths and are not able to separate channels in such an asymmetric fashion. Thus, in conventional WDM systems, either the pass bands must be made as wide as the widest information bandwidth or else the information bandwidth on each channel is restricted to the available hardware band pass. In either case, this can lead to inefficient use of overall optical system bandwidth.
Accordingly, there exists a need for an optical channel separation mechanism which would allow a wavelength division multiplexer to separate channels in an asymmetric fashion. The mechanism should allow the hardware band pass to correlate with the channel information bandwidth, and allow more efficient use of optical bandwidth in WDM systems in which signals with different data transfer rates propagate simultaneously. The present invention addresses such a need.